Molecular beam epitaxy process of making superconducting oxide thin films using an oxygen radical beam

ABSTRACT

A method makes a superconducting oxide thin film by irradiating an oxygen radical beam with necessary elements of the compound onto a substrate mounted in a molecular beam epitaxy system. The process can selectively form the superconducting oxide thin film on the substrate more efficiently in a direct reaction manner while maintaining the vacuum chamber of the molecular beam epitaxy system at a higher vacuum level.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process of selectively preparing ahigh-quality superconducting oxide thin film on the substrate of anelectronic superconducting device.

2. Description of the Related Art

In recent years, various superconducting oxide materials such as yttrium(Y), bismuth (Bi) and thallium (Tl) compounds have been discovered whichcan exhibit superconductivity at their critical temperature which ishigher than the temperature of liquid nitrogen. Studies are activelybeing undertaken to enable the application of thin films formed fromthese superconducting oxide materials to electronic superconductingdevices. In order to provide good device properties, it is firstnecessary to prepare a high-quality superconducting oxide thin filmhaving a smooth surface on a dielectric or semiconductor substrate.

The current methods of preparing the superconducting oxide thin filminclude sputtering, reactive co-evaporation, laser ablation, molecularbeam epitaxy (MBE) and MOCVD. Among them, MBE is a film forming processwhich is superior in film thickness at atomic layer level and incontrollability of crystalline structure. Such a process can prepare athin film having an excellent evenness.

In order to realize the molecular beam state of evaporating matter, theMBE process requires the maintaining of a high vacuum growth chamber sothat the mean free path of the evaporating matter will be maintainedsufficiently. When an oxide thin film is to be produced by the MBEprocess, sufficient oxygen must be supplied to the thin film whilemaintaining a high vacuum. Since the oxygen molecules are less reactive,an increased amount of oxygen must be introduced into the growth chamberso as to supply the necessary amount of oxygen into the thin film. Ifthe normal oxygen O₂ is introduced into the growth chamber, the degreeof vacuum in the growth chamber will be extremely deteriorated.Therefore active sources of oxygen such as ozone O₃, nitrogen dioxideNO₂ and the like are now used to oxidize the thin film more effectivelywith a minimum amount of oxygen which is introduced from out side of thegrowth chamber.

The effective oxidization of the thin film with ozone or nitrogendioxide is due to the fact that the active oxygen source can be easilydecomposed to form oxygen radicals (also called excited state, nascentstate and atomic state oxygen) which are very active for oxidization (O₃→O*+O₂, NO₂ →O*+NO). On the contrary, the non-decomposed oxygenmolecules are less active than the oxygen radicals.

However, O₂ and NO produced from ozone or nitrogen dioxidesimultaneously with the production of oxygen radicals will deterioratethe degree of vacuum in the growth chamber of the molecular beam epitaxysystem since they remain in the chamber. In order to decompose ozone ornitrogen dioxide, it is further necessary to obtain thermal energy fromthe substrate surface. Additionally, the ozone and nitrogen dioxide canbe reactive only after they have diffused to some extent on the surfaceof the substrate.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a processof selectively making a superconducting oxide thin film on a substratewith an increased efficiency and in a direct reaction manner bymaintaining the vacuum chamber of the molecular beam epitaxy system athigher degrees of vacuum.

To this end, the present invention provides a process of making asuperconducting oxide thin film, characterized by the step ofirradiating an oxygen radical beam with the necessary elements ofcomponents onto a substrate mounted on the molecular beam epitaxy systemto form a superconducting oxide thin film.

Oxygen radicals are produced from the decomposition of oxygen molecules(O₂ →20*). The oxygen radical beam is formed when the oxygen moleculesare decomposed before reaching the substrate. The oxygen radical beam isvery highly reactive since it is activated into its excited state. Thisprovides the direct reaction and is suitable for maintaining theincreased degree of vacuum since any remaining gas will not be createdsimultaneously with the production of the oxygen radicals, unlike ozoneand nitrogen dioxide.

The oxygen radicals are formed by decomposing the gaseous oxygen O₂ intoits atomic state under high-frequency discharge within a quartz tubewhich has a diameter equal to 30 mm and a length equal to 60 mm. Theoxygen radicals are then extracted outwardly from the quartz tubethrough a plurality of elongated openings formed therethrough andextending parallel to each other at the forward end of the quartz tubeunder a differential pressure between the inside and outside thereof.For example, each of such openings has a diameter equal to 0.3 mm and alength equal to 1.5 mm. Thus, the beam of oxygen radicals have improveddirectivity.

Since the oxygen radicals are extremely reactive, the oxide film can bemore effectively produced by irradiating the oxygen radical beam whilemaintaining the MBE system at an increased degree of vacuum. Thesufficient oxidization of thin film so formed provides an excellentsuperconducting oxide thin film without the need for any thermaltreatment which would insufficiently supply oxygen.

If a mask having a pattern required to produce a device is placedadjacent to a substrate to block a part of the beam so that the oxygenradical beam can be irradiated onto the substrate only at the necessaryparts, a superconducting oxide thin film having the desired pattern canbe selectively formed without the need for any subsequent heat treatingstep. As a result, the present invention is expected to provide one stepat which the thin film is deposited and patterned simultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a thin film forming system utilizing amolecular beam epitaxy device.

FIG. 2 is a schematic view of an oxygen radical producing device.

FIG. 3 is a schematic view illustrating an area adjacent to a substratewhich is being irradiated by the oxygen radical beam or the like.

FIG. 4 is a diagrammatic sketch of a photograph illustrating the surfaceof a thin film which comprises a substrate and oxidized and non-oxidizedfilm sections formed thereon.

FIG. 5 is a graph illustrating the electric resistance characteristic ina thin film of Bi-Sr-Ca-Cu-O formed in the first example of the presentinvention.

FIG. 6 is a diagrammatic view of a superconducting superlattice.

FIG. 7 illustrates crystalline structures of Bi-Sr-Ca-Cu-O.

FIG. 8 is a graph illustrating the x-ray diffraction pattern of asuperlattice.

FIG. 9 is a graph illustrating the resistance-temperaturecharacteristics of the superlattice structure of Bi-Sr-Ca-Cu-O.

FIG. 10 is a diagrammatic view of a superlattice film [(Bi₂ Sr₂ Cu₁O₆)_(n) /(Bi₂ Sr₂ Ca₁ Cu₂ O₈)₁ ]₁₅ formed in the third example of thepresent invention.

FIG. 11 is a diagrammatic view of a superlattice film [(Bi₂ Sr₂ Cu₁O₆)_(n) /(Bi₂ Sr₂ Ca₁ Cu₂ O₈)₄ ]₁₀ formed in the third example of thepresent invention.

FIG. 12 is a graph illustrating the x-ray diffraction pattern of thesuperlattice film [(Bi₂ Sr₂ Cu₁ O₆)_(n) /(Bi₂ Sr₂ Ca₁ Cu₂ O₈)₁ ]₁₅formed in the third example of the present invention.

FIG. 13 is a graph illustrating the x-ray diffraction pattern of thesuperlattice film [(Bi₂ Sr₂ Cu₁ O₆)_(n) /(Bi₂ Sr₂ Ca₁ Cu₂ O₈)₄ ]₁₀formed in the third example of the present invention.

FIG. 14 is a graph illustrating the resistance-temperaturecharacteristics of the superlattice film [(Bi₂ Sr₂ Cu₁ O₆)_(n) /(Bi₂ Sr₂Ca₁ Cu₂ O₈)₄ ]₁₀ formed in the third example of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described by referring to someexamples thereof and associated drawings.

EXAMPLE 1

Referring to FIG. 1, there is shown a thin film producing systemutilizing a molecular beam epitaxy device. This system comprises avacuum chamber 1 including five sources of components to be evaporatede, four of which are individually adapted to evaporate bismuth oxide(Bi₂ O₃), metal strontium (Sr), metal calcium (Ca) and copper (Cu),respectively. These evaporated components are then irradiated toward asubstrate 2 in atomic or molecular form.

The system also comprise a radical beam source 3 equipped in the vacuumchamber 1. The radical beam source 3 is adapted to produce oxygenradicals from molecule state oxygen (O₂) which is supplied from a supplyof oxygen 3a and irradiates oxygen radicals O* toward the substrate 2.

Although the substrate 2 is made of MgO (100) herein, it may be formedof SrTiO₃ (100) or YAlO₃ (001).

The substrate 2 is held by a substrate holder 2a within the vacuumchamber 1 which is maintained at a given vacuum level by means of aturbo pump and an ion pump. The interior of the vacuum chamber 1 is alsomaintained at a predetermined temperature level by a liquid Ncryoshroud. The substrate 2 can be loaded or unloaded to the vacuumchamber 1 through a loading chamber.

Only two sources of components to be evaporated e, for Bi₂ O₃ and Cu,are in the form of an electron gun while the remaining sources are inthe form of a crucible. The amount of evaporated component from each ofthese sources e is individually detected by the respective one of thedetectors d. Based on the detected data, a film deposition controllermay control the sources e.

An electron gun functions to irradiate an electron beam onto thesubstrate 2, the crystal structure of which is then investigated by aRHEED screen.

FIG. 2 shows the schematic diagram of an oxygen radical producing devicewhich comprises a quartz tube 4 having a diameter equal to 30 mm and alength equal to 60 mm. Oxygen O₂ is introduced into the quartz tube 4and dissociated into oxygen radicals under high-frequency excitationfrom a high-frequency coil 5. Oxygen radicals produced from thedissociation of oxygen molecules are drawn out of the quartz tube 4,that is, into the interior of the vacuum chamber 1 through 37 elongatedopenings 6 formed in the quartz tube at its tip end under a differentialpressure between the inside and outside of the quartz tube. Each of theopenings 6 has a diameter of 0.3 mm and a length of 1.5 mm. This canrealize a beam of oxygen radicals improved in directivity, which will bedirected toward the substrate 2.

It is suitable that the distance between the tip or forwardmost end ofthe quartz tube 2 and the substrate 2 is equal to 100 mm.

The high-frequency coil 5 receives a high-frequency current whosefrequency is equal to 13.56 MHz, which is generated by an RF generatorthrough an RF match unit.

The amount of gaseous oxygen O₂ supplied to the quartz tube 4 may beadjusted by a valve.

FIG. 3 shows a schematic diagram of an area adjacent to the substrate 2onto which the bismuth oxide Bi₂ O₃, Sr, Ca and Cu in the atomic ormolecular state and the oxygen radicals are irradiated. As will beapparent therefrom, the irradiation of the oxygen radicals O* arecontrolled by means of a mask 7. That portion of the substrate behindthe mask 7 will not be oxidized.

In such a manner, a thin film of Bi-Sr-Ca-Cu-O will be formed byirradiating Bi₂ O₃, Sr, Ca and Cu and the oxygen radicals O* onto theheated substrate 2. The components Bi₂ O₃, Sr, Ca Cu are sequentiallydeposited onto the substrate 2 in the following sequence:Bi-Sr-Cu-Ca-Cu-Sr-Bi.

The rate of evaporation of each of the components was substantially asfollows:

Bi₂ O₃ : 0.125 nm/second

Sr: 0.220 nm/second

Cu: 0.040 nm/second

Ca: 0.220 nm/second

O*: 1×10¹⁶ species/second cm² (irradiation rate)

The opening time for each component source to be evaporated (i.e. timefor which the respective shutter is opened in a sequence) was asfollows:

Bi₂ O₃ : 3.02 seconds

Sr: 4.02 seconds

Cu: 2.90 seconds

Ca: 3.85 seconds

Although the opening and closing of the shutter are repeated 100 timesin the example 1, any number of opening and closing cycles may beselected as long as it is at least 10 cycles.

Thin films of Bi-Sr-Ca-Cu-O having a thickness ranging between 160 nmand 250 nm were obtained when the temperature of the substrate was inthe range between 650° C. and 750° C. and the growth rate of the thinfilm was in the range between 0.06 nm/second and 0.20 nm/second.

Although the components Bi₂ O₃, Sr, Ca and Cu and the oxygen radicals O*were simultaneously irradiated onto the substrate 2 in the example 1,the oxygen radicals O* may be irradiated onto the substrate 2 after eachdeposition in the sequence of Bi-Sr-Cu-Ca-Cu-Sr-Bi.

FIG. 4 shows the diagrammatic view of a photograph illustrating thestate of the boundary (on the film surface) between an oxidized filmportion (thin film of Bi-Sr-Ca-Cu-O) and a non-oxidized film portion(thin film of Bi-Sr-Ca-Cu), which are formed on the substrate 2. As willbe apparent from this photograph, the width of the boundary area 10between the non-oxidized and oxidized film portion 8 and 9 is verynarrow, in the range between 10 μm and 20 μm. This results from theoxidization by the direct reaction of the oxygen radicals. FIG. 4 alsoshows the fact that the oxygen radical beam has improved directivity.

On measuring the resistance and temperature characteristics of theBi-Sr-Ca-Cu-O thin film 9 formed in the oxidized film portion, such agraph as shown in FIG. 5 was plotted. This thin film exhibitedsuperconductivity in the as-deposited sate without any heat treatment.The temperature with zero resistance Tc (end) was 75 K. The non-oxidizedfilm portion 8 did not exhibit superconductivity. When the x-raydiffraction pattern of the thin film was measured to investigate thecrystalline structure thereof, the composition of the oxidized filmportion was Bi₂ Sr₂ Ca₁ Cu₂ O_(y). On the other hand, the definitestructure of the non-oxidized film portion was not found. Although theexample 1 has been described as to the process of making thesuperconducting oxide thin film of Bi components, the present inventionmay be applied to other processes of making superconducting oxide thinfilms of Y(rare-earth-metal) based and Tl based compounds. In the lattercase, for example, the temperature of the substrate 2 may be in therange between 550° C. and 700° C.

EXAMPLE 2

By changing the sequence of each of the shutters located in therespective sources of component to be evaporated, which is opened andclosed, a superconducting superlattice was formed which comprises analternate lamination of nonsuperconducting layers (Bi₂ Sr₂ Cu₁ O₆) andsuperconducting layers (Bi₂ Sr₂ Ca₁ Cu₂ O₈), as shown in FIG. 6. If itis assumed that the nonconducting layer is layer N and thesuperconducting layer is layer S, the superconducting superlattice had aS-N-S structure which was repeated as shown by S-N-S-N-S . . .S-N-S-N-S. Namely, this would provide a S-N-S array along C-axisdirection.

The components Bi₂ O₃, Sr, Ca and Cu and the oxygen radicals O* wereirradiated onto the heated substrate 2 to form a superlattice structureof Bi-Sr-Ca-Cu-O. If it is to form a nonsuperconducting layer (Bi₂ Sr₂Cu₁ O₆), Bi₂ O₃, Sr and Cu will be sequentially deposited in thesequence of Bi-Sr-Cu-Sr-Bi. This sequence is represented by sequence Aand it corresponds to 2201 phase (FIG. 7).

On the other hand, if it is to form a superconducting layer (Bi₂ Sr₂ Ca₁Cu₂ O₈), BiOx, Sr, Ca and Cu will be sequentially deposited in thesequence of Bi-Sr-Cu-Ca-Cu-Sr-Bi. This sequence is represented bysequence B and it corresponds to 2212 phase (FIG. 7).

On the growth of a thin film, sequence A and B are alternately repeatedto form a superlattice structure consisting of an alternate laminationof nonsuperconducting phases (Bi₂ Sr₂ Cu₁ O₆) and superconducting phases(Bi₂ Sr₂ Ca₁ Cu₂ O₈).

In the example 2, the amount of each of the evaporated components wassubstantially as follows:

Bi₂ O₃ : 0.100 nm/second

Cu: 0.020 nm/second

Sr: 0.100 nm/second

Ca: 0.100 nm/second

O*: 1×10¹⁶ species/second cm² (irradiation rate)

The opening time for each component source to be evaporated (i.e. timefor which the respective shutter is opened in a sequence) was asfollows:

Bi₂ O₃ : 3.20 seconds

Cu: 3.80 seconds

Sr: 3.40 seconds

Ca: 3.50 seconds

(These time periods are at the substrate temperature which will bedescribed below. They will be in the range between about 3.00 secondsand about 4.00 seconds even if the temperature of the substrate isvaried.)

The other conditions on production were as follows:

Growth rate: 0.01-0.10 nm/second

Temperature of Substrate: 600°-750° C.

Film thickness: 20-200 nm

Material of substrate used: MgO (100) or SrTiO₃ (100) or YAlO₃ (001)

Under the above conditions, a superlattice of Bi-Sr-Ca-Cu-O was producedwhich comprises an alternate lamination consisting of sixnonsuperconducting layers (Bi₂ Sr₂ Cu₁ O₆) each having a thickness equalto 36 angstroms and six superconducting layers (Bi₂ Sr₂ Ca₁ Cu₂ O₈) eachhaving a thickness equal to 30 angstroms. In this case, the sequence Awas repeated three times to form each nonsuperconducting layer while thesequence B was repeated two times to form each superconducting layer.Namely, the sequence A and B were repeatedly arranged as shown by(AAABB)×6.

The x-ray diffraction pattern of the superlattice so formed is shown inFIG. 8. This pattern corresponds to 6.6 nm of the long period. Satellitediffraction curves (i.e. lower peaks on the both sides of each of themain diffraction curves) are also observed as shown by arrows. It isthus understood that a single crystal thin film of the superlatticestructure comprising an alternate lamination consisting ofnonsuperconducting layers each having its thickness equal to 36angstroms and superconducting layers each having its thickness equal to30 angstroms is formed on the substrate 2.

The measurements of the resistance-temperature characteristics of theformed Bi-Sr-Ca-Cu-O superlattice are shown in FIG. 9. When the thinfilm having such a superlattice was electrically energized by anelectric current flowing therein parallel to the film surface andmeasured with the dependency of the electric resistance on thetemperature, it exhibited superconductivity with the superconductivitytransient temperature Tc(onset) being 90K and the zero resistancetemperature Tc(end) being in the range between 35K and 50K.

The superconducting layers of Bi₂ Sr₂ Ca₁ Cu₂ O₈ may be replaced bysuperconducting layers of Bi₂ Sr₂ Ca₂ Cu₃ O₁₀. In this case, thesequence of deposition is Bi-Sr-Cu-Ca-Cu-Ca-Cu-Sr-Bi. This correspondsto 2223 phase (FIG. 7).

EXAMPLE 3

As in the example 2, a superlattice structure was formed which comprisesan alternate lamination consisting of nonsuperconducting layers (Bi₂ Sr₂Cu₁ O₆) and superconducting layers (Bi₂ Sr₂ Ca₁ Cu₂ O₈). By repeatingeach of the sequences A and B a number of times for each deposition, thethickness of each of the nonsuperconducting and superconducting layerscan be controlled.

In other words, the alternate lamination of the example 3 comprising thenonsuperconducting layers N and the superconducting layers S is similarto that of the example 2, but the example 3 is characterized by therepetition of each of the nonsuperconducting and superconducting layersN and S a number of times for each deposition. Thus, the example 3provides a superlattice structure comprising (N)n(S)m(N)n(S)m(N)n(S)m .. . although the example 2 provides a superlattice structure comprisingNSNSNS . . . Since the above subscripts n and m can be freely changed bysuitably setting the conditions, one can provide an alternate laminationconsisting of nonsuperconducting and superconducting layers each havingthe desired thickness.

In the example 3, the amount of each of the evaporated components wassubstantially as follows:

Bi₂ O₃ : 0.100 nm/second

Cu: 0.020 nm/second

Sr: 0.100 nm/second

Ca: 0.100 nm/second

O*: 1×10¹⁶ species/second cm² (irradiation rate)

The opening time for each component source to be evaporated (i.e. timefor which the respective shutter is opened) was as follows:

Bi₂ O₃ : 3.30 seconds

Cu: 3.65 seconds

Sr: 3.50 seconds

Ca: 3.30 seconds

(These time periods are at the substrate temperature which will bedescribed below. They will be in the range between about 3.00 secondsand about 4.00 seconds even if the temperature of the substrate isvaried.)

The other conditions on production were as follow:

Growth Rate: 0.01-0.10 nm/second

Temperature of Substrate: 600°-750° C.

Film thickness: 20-200 nm

Material of substrate used: MgO (100) or SrTiO₃ (100) or YAlO₃ (001)

By changing the conditions on production in such a manner, the thicknessof each of the nonsuperconducting and superconducting layers and theirratio of thickness can be controlled freely.

After formation, the thin film was naturally cooled to 300° C. for about15 minutes while irradiating the oxygen radical beam. This is to preventthe desorption of oxygen from the thin film immediately after beingformed. The composition of the formed film is analyzed by the use of anEPMA (electron probe microanalyser) which can observe thetwo-dimensional atomic distribution by scanning the surface of aspecimen with an electron beam. The crystalline structure is estimatedby RHEED (reflection high energy electron diffraction) and XRD (x-raydiffraction).

Under the above conditions, a superlattice structure was formed whichconsisted of 10-15 nonsuperconducting layers (Bi₂ Sr₂ Cu₁ O₆) and 10-15superconducting layers (Bi₂ Sr₂ Ca₁ Cu₂ O₈).

A specimen having Bi-components superlattice can be grown by thelayer-by-layer MBE process, but the example 3 produced two types ofspecimens each of which comprised one superconducting layer or foursuperconducting layers.

The structure of the one superconducting layer type specimen is

    [(Bi.sub.2 Sr.sub.2 Cu.sub.1 O.sub.6).sub.n /(Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.8).sub.1 ].sub.15 (n=2-5),

as shown in FIG. 10.

The structure of the four superconducting layer type specimen is

    [(Bi.sub.2 Sr.sub.2 Cu.sub.1 O.sub.6).sub.n /(Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.8).sub.4 ].sub.15 (n=1-5),

as shown in FIG. 11.

When the one superconducting layer type specimen was being formed, thesequence of opening and closing in the source shutters (shuttersequence) is n times for the sequence A and one time for the sequence B,with such a cycle being repeated fifteen times. When the foursuperconducting layer type specimen was being formed, the sequence ofopening and closing in the source shutters (shutter sequence) is n timesfor the sequence A and four times for the sequence B, with such a cyclebeing repeated fifteen times.

All the RHEED patterns (not shown) of the respective specimen surfaceswere of a streak type which exhibited the epitaxial growth. In order toconfirm the production of the long period structures corresponding tothe shutter sequences, the x-ray diffraction patterns of the thin filmswere observed as shown in FIGS. 12 and 13. This observation discoveredsatellite diffraction curves (i.e. lower peaks on the both sides of eachof the main diffraction curves) which corresponded to the long periodstructures caused by multi-layers. From this fact, it is understood thatthe spacing of the lower peaks is decreased more as the cycle isincreased.

On calculating the long periods from these diffraction curves, the onesuperconducting layer type film has the long periods equal to 3.98,5.22, 6.45 and 7.68 nm while the four superconducting layer type filmhas the long periods equal to 7.4, 8.6, 9.8, 11.0 nm. This means thatthe long periods are increased by about 1.23 nm which corresponds to 1/2unit cell in the 2201 phase. From this fact, it is understood that thechanging of the shuttering sequence can produce a superlatticeconsisting of 2201 phase and 2212 phase. Further, this shows that a thinfilm controlled in phase in the C-axis direction can be produced by each1/2 unit cell.

FIG. 14 shows the resistance-temperature characteristics of theBi-Sr-Ca-Cu-O superlattice (of four superconducting layers) which weremeasured by flowing the electric current in the film parallel to thefilm surface with the conventional four terminal measuring process. Thefact that Tc(end) is substantially not variable and remains at about 50Keven if there are changes of n=0, n=3 and n=5 shows that Tc(end) isinvariable even if the thickness of the 2201 layer (nonsuperconductinglayer) is variable. Namely, the thickness of each of the layers isregularly controlled for each layer.

On the other hand, the one superconducting layer type film (not shown)has Tc(onset) equal to 70K even if it has two-four nonsuperconductinglayers. This exhibits that the thin film was formed into a regularlamination configuration without any disturbance. It is expected thatsuch a thin film can be used as a device having better characteristics.

Since the process of the present invention for producing asuperconducting oxide thin film is characterized by the fact that thesuperconducting oxide thin film can be formed by irradiating the oxygenradical beam with the elements of the compound of the superconductingoxide thin film onto the substrate, a higher degree of vacuum can bemaintained with the direct reaction being realized more efficiently. Thesuperconducting oxide thin film can be selectively formed on thesubstrate by the use of the oxygen radical (nascent state oxygen) beamwithout any subsequent heat treatment.

In accordance with the present invention, furthermore, a superlatticestructure can be formed which comprises an alternate laminationconsisting of nonsuperconducting and superconducting layers in anysequence of deposition, each of these layers having the desiredthickness.

We claim:
 1. A method of making a superconducting oxide thin film by theuse of a molecular beam epitaxy process (MBE process), said methodcomprising the steps of:(a) forming molecular beams from the elements ofa compound of the superconducting oxide thin film; (b) forming an oxygenradical beam in a tube made of insulating material and having an outputend provided with a plurality of openings; and (c) irradiating saidoxygen radical beam through the apertures and with said molecular beamsonto a substrate mounted in a molecular beam epitaxy system to form saidsuperconducting oxide thin film.
 2. A method as defined in claim 1,further comprising the step of controlling the amounts of said oxygenradical beam and molecular beams to control the composition of the thinfilm formed on said substrate, thereby forming a nonsuperconductingoxide thin film.
 3. A method as defined in claim 2 wherein a superlattichaving an alternate lamination consisting of superconducting andnonsuperconducting layers is formed on said substrate by alternatelydepositing said superconducting and nonsuperconducting layers on saidsubstrate.
 4. A method as defined in claim 3 wherein the thickness ofeach of said superconducting and nonsuperconducting layers in saidsuperlattice is individually varied to form said superlattice.
 5. Amethod as defined in claim 4 wherein said layer thickness controllingstep includes controlling the time of irradiation for said molecularbeams of the elements of the compound of said superconducting oxide andthe time of irradiation for said oxygen radical beam.
 6. A method asdefined in claim 1 wherein said oxygen radical beam forming stepincludes dissociating oxygen atoms under high-frequency discharge toproduce oxygen radicals.
 7. A method as defined in claim 6 wherein saidmolecular beam forming step includes evaporating the elements of thecompound of the superconducting oxide.
 8. A method as defined in claim7, further including the step of blocking at least one of the molecularbeams for a given time period.
 9. A method as defined in claim 8 whereinthe elements of the compound of said superconducting oxide are selectedfrom the group consisting of Bi, Sr, Ca, Cu, Y, Ba, Tl and O.
 10. Amethod as defined in claim 1, wherein the tube is a quartz tube.
 11. Amethod as defined in claim 1, further comprising the step of disposing amask between the tube and the substrate in order to prevent oxygenradicals in the beam from striking a part of the substrate, whereby anon-superconductive area is formed on that part of the substrate.